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Discussion Starter · #1 · (Edited)
New to mountain bikes. But one question I had for a while is how to compare bike geometries. I know this is a broad topic, but today we can focus on sizes - gauging bike sizes from geometry measurements.

For example:

2021 Specialized Fuse M size has the measurements:
Reach: 440mm
Stack: 625mm
Top tube: 619mm
Reference: Fuse Comp 29 | Specialized.com

2021 Treck X-Caliber ML (medium large) size has the measurements:
Reach: 434mm
Stack: 598mm
Top tube: 617mm
Reference: X-Caliber 8 | Trek Bikes

Question:
Isn't reach/stack/tube the most important measurements to determe a bike's size? Then how can the Treck ML has smaller numbers than Specialized M? The Treck ML is supposed to fill the gap between M and L. And based on my height, I am in the middle between a Specialized M and L, but right fit for Trek ML, which means, ML is supposed to be larger than M.
 

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Like with everything else, bike sizes are determined by the manufacturer, so they vary across brands. If you want to compare quickly check out 99spokes.com
 

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Go to 99spokes and pick the bikes you want to compare. IIRC they mock up the frames overlapping each other so you can compare exactly how they differ.

Also, effective top tube is an important measurement when comparing sizes.
 

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There are three important numbers to use when comparing bike sizing, the letter sizing designation between manufactures tells you nothing:
  • Reach- This tells you how far the center of the headtube at the top of the headtube is the center of the bottom bracket. It determines in large part, how stretched out you feel when standing on the pedals.
  • Stack- How much higher the top of the headtube is than the bottom bracket. This tells you approximately how high the bars will feel when standing on the pedals and seated.
  • Effective Toptube- This will tell you how stretched out you feel when seated on the bike.
Very important- These numbers are a starting place. Seat angle, stem length, headtube length and saddle positioning will effect how a bike feels seated and standing. Short headtubes can be accommodated for by running spacers under the stem, but when you do this, for every 10mm of spacers you raise the stack about 8mm and shorten the reach by about 4mm. Also, not all manufactures measure these exactly the same (they should, but don't).

So use the three above measurements to get an idea of if a bike is going to be sized right for you, but realize that without throwing a leg over the bike, you could find that the actual fit is a bit different than the numbers suggest.

Also for comparing, I like Geometry Geeks
 

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The intention of the two bikes you mentioned are completely different, IMO. The first clue would be the head tube angles. Looks easy from here has the right idea in pointing you to 99spokes. Geometry Geeks Cary linked to is also great, but I have found I often have to provide my own geometry information.

To make things easier, here is a link to the specific bikes you are asking about. Unfortunately, they don't have graphics for both bikes to compare but the numbers are there.

Compare: 2021 Specialized Fuse Comp 29 vs Trek X-Caliber 8 – 99 Spokes
 

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Discussion Starter · #9 ·
The intention of the two bikes you mentioned are completely different, IMO. The first clue would be the head tube angles. Looks easy from here has the right idea in pointing you to 99spokes. Geometry Geeks Cary linked to is also great, but I have found I often have to provide my own geometry information.

To make things easier, here is a link to the specific bikes you are asking about. Unfortunately, they don't have graphics for both bikes to compare but the numbers are there.

Compare: 2021 Specialized Fuse Comp 29 vs Trek X-Caliber 8 – 99 Spokes
I know they are different so we do not compare angle. Here the focus is size. If you take a look of Trek Rescoe, the ML 3 numbers are also smaller then M Fuse.
 

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I know they are different so we do not compare angle. Here the focus is size. If you take a look of Trek Rescoe, the ML 3 numbers are also smaller then M Fuse.
I'm not sure you understand what I am saying. The head angle can be a good way to get an indication of the intended use of the bike. The slacker the angle, the more it is meant to go downhill (trail, all mountain, enduro). A steeper angle would indicate that the bike is more geared toward cross country or casual riding. The intent of the bike will also affect other aspects of geometry (reach, stack, effective top tube, rear center, etc). That is why I made the comment about the intent of the bike. Unless the intent of the bikes is the same, you are going to see some significant geometrical differences.

The Fuse/Roscoe comparison is better, but you can look at the bikes and see that Fuse has more modern (aggressive?) geometry. Longer. Lower. Slacker. Also, the discrepancy in bike sizing (in many cases a holdover from road bikes) is the reasons guys like Lee McCormack have come up with new fitment methods like RAD. Bottom line? Don't worry about a manufacturer's stated size. Worry about how the bike fits YOU. This can be tough with bike shortages and not being able to throw a leg over a bike. But systems like Lee's seek to resolve that with a little bit of math that should at least get you into the ballpark. Though you size yourself according to a method like RAD, it doesn't mean that all bikes with the best fit are going to ride the same. Even in the same category. The rest of the geometry still matters (HA, SA, trail, RC, etc).
 

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Like with everything else, bike sizes are determined by the manufacturer, so they vary across brands. If you want to compare quickly check out 99spokes.com
Not just brands but models within a brand.
I'm currently on a Trek E-Caliber in a Small, but if I were to get a Rail, I'd probably want a Medium.
 

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Don't geek out on the numbers. I'm guilty of that myself, and have the spreadsheets to prove it. :) While nerding over the numbers in my initial researches to pick a bike, I admit that my numbers usually go out the window once I actually get on the bikes and ride them. What looked perfect on paper wasn't what worked the best on the trail.
 
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